1. Field of the Invention
This invention relates generally to microwave frequency filters, and relates more particularly to filters utilizing thin-film lumped elements, including spiral inductors and capacitive pi networks.
2. Description of the Relevant Art
In the past, filters designed for operation in microwave frequency ranges, in excess of about 500 MHz, include designs such as discrete lumped element filters, mechanical distributed element filters, and printed distributed element filters. Discrete lumped element filters use separately fabricated air-wound inductors and parallel-plate capacitors that are wired together into a filter circuit. Discrete lumped element filters require significant amounts of hand labor to build and tune. The reliability of such filters is highly dependent on the skill and care taken by the assembler. The volume occupied by these kinds of filters is small but significant.
Mechanical distributed filters use coupled bars or rods to form transmission line networks which make a filter circuit. The length dimensions of the coupled bars or rods are typically one-quarter or one-half of the wave length at the center frequency of the filter. Mechanical distributed filters occupy a significant volume, particularly at lower microwave frequencies. Another disadvantage is that spurious responses, occurring at multiples of the center frequency, degrade the performance of the filter in the stop band.
Printed distributed element filters have a single layer of metal traces printed onto an insulating substrate backed with a ground plane. Similar to the bars or rods of mechanical filters, these traces form transmission line networks which make up the filter circuit. The length dimensions of these printed traces are one-quarter to one-half of the wave length at the center frequency of the filter. At lower microwave frequencies, these filters are large in size. Another disadvantage to printed distributed filters is that spurious responses that occur at multiples of the center frequency will degrade the performance of the filter in the stop band.